Improvements in cleaning electrophotographic imaging surfaces

ABSTRACT

In an electrophotographic reproduction process in which an electrostatic image is formed on a surface, developed with powder particles, the particle image transferred to a reception material and the surface re-used to form further images, the cleaning of the surface as with a brush or fibrous web before re-use of the surface is facilitated by applying to the surface at least one hydrophobic bivalent or trivalent metal salt of a half ester of a branched-chain or straight-chain aliphatic dicarboxylic acid or of a mono- or di-ester of a phophorus oxyacid. The metal salt can be deposited on the surface prior to formation of any image thereon or after transfer of the powder particle image.

United States Patent [1 1 Van Engeland et a1.

22 Filed: May 19,1972 [21] Appl. No.: 254,903

[30} Foreign Application Priority Data May 21, 1971 Great Britain 16346/71 [52] US. Cl 96/1 R, 117/175, 355/15, 134/7 [51] Int. Cl C03g 13/08 [58] Field of Search 117/175; 96/1 R; 118/637; 355/15, 17; 134/7 [56] References Cited UNITED STATES PATENTS 2,772,991 12/1956 Insalaco l17/17.5

[111 3,793,018 [451 Feb. 19, 1974 -Palcrmiti et a1. 1l7/l7.5

Primary ExaminerMurray Katz Assistant ExaminerM. Sofocleous Attorney, Agent, or Firm-William J. Daniel 57 ABSTRACT? In an electrophotographic reproduction process in which an electrostatic image is formed on a surface, developed with powder particles, the particle image transferred to a reception material and the surface reused to form further images, the cleaning of the surface as with a brush or fibrous web before re-use of the surface is facilitated by applying to the surface at least onehydrophobic bivalent or trivalent metal salt of a half ester of a branched-chain or straight-chain aliphatic dicarboxylic acid or of a monoor di-ester of a phophorus oxyacid. The metal salt can be deposited on the surface prior to formation of any image thereon or after transfer of the powder particle image.

16 Claims, No Drawings l IMPROVEMENTS IN CLEANING The present invention relates to electrophotography and more particularly to improvements in the formation of images by electrophotographic processes.

Known. electrophotographic processes for producing visible images comprise the steps of electrostatically charging in the dark a photoconductive surface of an inorganic photoconductor, e.g. zinc oxide and selenium,or of an organic photoconductor, image-wise'exposing the said surface to form a latent electrostatic image and developing thematerial to form a visible image by depositing on the image a finely divided electroscopic material usually. aresin, which may becoloured and is known as toner. Depending on the sign of the electrostatic charge the toner particles are attracted and deposited on the charged areas of the latent image or are repelled by the charged areas and deposited on the discharged areas. The toner image is then fixed by heating, by an overcoating treatment or by the action of solvents. Before fixing, the toner image may be transferred to a support surface such as paper and then permanently affixed thereto. Instead of forming the latent electrostatic image by the steps described above it is also possible to directly charge the photoconductive layer in image configuration.

For a better control of the-development of the latent image the toner is used in combination with solid carrier particlesor the toner particles can be dispersed in an insulating liquid.

If a dry developer is used, it is composed of two components, a finely ground pigmented or coloured resinous toner and a relatively coarse-grained carrier material. For the development of the latent image, the developing mixture may be cascaded merely over the exposed plate. The carriermaterial, e.g.,glass or steel beads, which may be enveloped by a resinous filmforming product, carries the toner as it cascades over the plate and also triboelectrically charges the toner particles to the desired polarity. As the toner carrier mixture flows over the surface bearing the latent electrostatic image thetoner particles are'attracted by the charged areas of the image and not by the discharged areas or background areas of the image. Most of the toner particles accidentally deposited on the background areas of the image, are taken away by the rolling carrier particles due to the greater electrostatic at traction between the toner and the carrier than between the toner and the discharged background. The powder image formed is then fixed as described above on the photoconductive plate or can be transferred to a receptor surface, e.g. a paper sheet. The transfer can be accomplished by bringing the powder image in contact with the receptor surface, if necessary, in the presence of an electrostatic field. If the receptor surface, e.g. the paper sheet, is then stripped from the image-carrying surface, it carries with it a substantial amount of toner particles in-the form 'ofthe desiredim age. Subsequently, this image can be made permanent, i.e. fixed, according to any desired method such as heating or solvent fixing.

. 2- I closed, e.g.,in U.S. Pat. specification No. 2,874,063 of Harold G. Greig issuedFeb. 2, 1959.

In electrophotographic imaging processes it is known to employ photoconductive plates in the form of a drum, which continuously rotates through a cycle of sequential operations including charging, exposure, development, transfer and cleaning. During development of the electrostatic latent image a powder image is formed on the photoconductive plate, which powder image is then transferred electrostatically to a receptor surface. Usually a residual .powder image remains on the plate after transfer. Therefore, before the plate is reused for the formation of a subsequent latent electrostatic image it is necessary to remove the residual pow der to prevent staining of the subsequent transfer image formed. Removal of the residual powder image occurs by contact of the plate comprising the residual powder image with a cleaning device, which includes brush type cleaning means as disclosed, e.g., in U.S. Pat.

specification No. 2,832,977 of Lewis E. Walkup and Herbert E. Carlton issued May 6, 1958 and web type cleaning means as disclosed, e.g., in U.S. Pat. specification No. 3,186,838 of William P. Graff, Jr. and Robert W. Gundlach issued June 1, 1965. Removal of the residual powder on the plate by means of a brush type cleaning means is usually effected by brushing the powder by means of one or more rotating brushes into a stream of air, which is exhausted through a filtering sys tem. The web type cleaning means comprises a web of fibrous material, which is advanced into pressure and rubbing contact with the plate surface.

Cleaning of reusable photoconductive plates or V drums is not always satisfactory, which may depend on a whole series of factors, e.g. the nature of the tonercarrier mixture employed and the relative humidity of the atmosphere. When'relative humidity is high the electrical and transfer characteristics of the photoconductive imaging surfaces are generally adversely effected and proper cleaning is impaired. Some toners and carriers are abrasive in nature so that on contact thereof with the imaging surfaces mutual deterioration of these components is accelerated. The cleaning devices also have an abrasive influence on the imaging surfaces when contacted therewith. Through the erosion of the imaging surfaces, cleaning becomes more difficult to effect.

According to the present invention an electrophotographic imaging process is provided, which comprises the steps of: forming a latent electrostatic image on an imaging surface; forming a powder image corresponding to said electrostatic image by applying a developing material to said surface, transferring said powder image from said imaging surface to a receptor surface, and. cleaning the said imaging surface, wherein at least one hydrophobic bivalent or trivalent metal salt of a halfester of a branched-chain or straight-chain aliphatic dicarboxylic acid and/or at least one bivalent or trivalent metal salt of a monoester or diester of a phosphorous oxyacid is-or are applied to said imaging surface before and/or during said cleaning thereof.

The application to the electrostatographic imaging surface of one or more metal salts as defined above not only facilitates the usual cleaning carried out between use and reuse of the imaging surface in that removal of residual toner particles from the imaging surface is promoted but also reduces abrasion of the photoconductive imaging surfaces, reduces staining of the elec- 3 trophotographic images obtained on the receptor materials and yields images with improved contrast.

When applying one or more said metal salts to the imaging surface before forming the latent electrostatic image or before development thereof, it is considered advisable to leave a deposit of such salt(s) on such surface to facilitate the later cleaning of the surface. Such a deposit can remain effective for any required length of time and the invention includes any electrophotographic imaging process involving the formation on a surface of an electrostatic image, its development by means of a powder, andsubsequent image-wise transfer of powder from said surface to a receptor material, wherein the surface on which said electrostatic image is formed is a surface on which there is a deposit of one or more metal salts as above defined. The imaging surface used in such a process can be very readily cleaned without abrading the surface.

A said metal salt or salts may be applied to the imaging surface in any convenient manner prior to forming the latent electrostatic image or subsequent to transferring the powder image from the imaged surface to the receptor surface. Such salt(s) may be applied to ,the imaging surface as loose powder, a melt, a solution, an emulsion or dispersion, etc. Loose powder can be sprinkled on the imaging surface and liquids containing the metal salts in dissolved or dispersed form may be applied to the imaging surface according to any conventional coating method, e.g., spraying, dip-coating etc. The metal salts are preferably dissolved or dispersed in a suitable solvent and the solution is then applied to the imaging surface whereupon'the solvent is removed by evaporation. In analogy to cascade development the metal salts may also be cascaded in the form of powder over the imaging surface.

According to a preferred embodiment of the invention, said metal salt(s) is or are applied to the imaging surface after transfer of the powder image to the recep-- tor surface to promote cleaning of the surface at that stage. In any case, the salt(s) may be applied by rubhing or wiping said surface for a certain period of time with an applicator device or member carrying such salt(s) on at least the side thereof which contacts such surface. The said applicator device or member may be used solely for treating the photoconductive plate,

, drum or other member providing the imaging surface with the metal salt(s) but it is preferably also used for cleaning the re-usable photoconductive surface as referred to above. Thus, application of the metal salt(s) to the imaging surface and cleaning of the surface between use and re-use may be combined in a single operation. In some cases the salt(s) applied to the imaging surface may be completely wiped away but a feature which is at present considered likely to be of importance comprises the cleaning of the imaging surface so that a deposit of the salt(s), which may be a very slight deposit, remains on the surface. This is of course proposed primarily with a view-to re-use of the surface in the formation of a further electrophotographic image. In the case that the process is performed cyclically, e.g., using a revolving drum with a photoconductive surface, cleaning by means of or with the aid of a said salt or salts may take place periodically, e.g., in every cycle or at intervals of two or more cycles.

The metal salt(s) for use according to the present invention should be hydrophobic i.e., substantially insoluble in water, otherwise they would be adversely afgeneral comprise a straight-chain or branched-chain aliphatic hydrocarbon group of at least five carbon atoms, preferably at least 10 carbon atoms, which group may be substituted with hydrophobic groups, e.g. halogen, preferably fluorine and/or may be interrupted by hetero atoms, e.g.' oxygen, sulphur and nitrogen. This group may be an ester group of the said metal salts or a substituent on the chain linking the two carboxylic acid groups of the dicarboxylic acid. In the phosphorus oxyacids preferably all the hydrogen atoms that wereplaceable by a metal atom are replaced by a metal atom.

Typical dicarboxylic acids from which the hydrophobic metal salts may be derived include: succinic acid, glutaric acid, maleic acid and glutaconic acid and mixtures thereof. The phosphorus oxyacids include ortho-, pyroand hypophosphoric acid, phosphonic acids and phosphinic acids. Excellent results are obtained where the metal salts are zinc salts. However, other salts may also be used, for example magnesium salts, calcium salts, strontium. salts, barium salts, iron salts, cobalt salts, nickel salts, copper salts, cadmium salts, aluminium salts and lead salts.

The preparationof the metal salts of the esters of phosphorus oxyacids. as defined is quite simple. According to a particularly suitable method a stoichiometric amount of oxyacid ester is dissolved in an aliphatic alcohol and added to a solution or suspension of a metal acetate inan aliphatic-alcohol. The metal salt of the phosphorus oxyacid ester normally separates by precipitation in the alcohol or is obtained therefrom by evaporation of the solvent.

The following preparations'illustrate how metal salts of phosphorus oxyacid esters were prepared.

PREPARATION 1 V COPPER ,(II) LAURYL PHOSPHONATE 4 g of copper(ll) acetate monohydrate were dissolved in 250 ml of methanol at 50 C. On adding to the warm solution 5 g of lauryl phosphonic acid, the copper(ll) lauryl phosphonate precipitated. The precipitate was then washed with methanol of 50 C.

% of Cu calculated 20.38

% of Cu found 20.35

PREPARATION 2 ZINC MONO(2-BUTYLOCTYL)-PHOSPI-IATE a. mono(2-butyloctyl')phosphoric acid A solution of 2 moles of 2-butyloctanol in 500 ml of dichloroethane was added dropwise with stirring to a solution of 2 moles of phosphorus oxychloride and 2.05 moles of pyridine in 500 ml of dichloroethane whilst maintaining the temperature at about 22 C. The pyridine hydrochloride formed was filtered with suction and the filtrate concentrated by evaporation. The residual oil was mixed with 1 litre of water and heated for 2 hours with stirring on a boiling water-bath. The mixture was then extracted with benzene, whereupon the latter was evaporated under reduced pressure. The residue obtained was completely degased by means of an oil pump.

Yield b. zinc mono( 2-butyloctyl) phosphate ether.

A solution of 1 mole of zinc acetate dihydrate in 75.0.

ml of methanol at 55 C was added to a solution -of-0.7 mole of mono(2-butyloctyl)phosphoric acid in 500ml of methanol. The zinc salt precipitated and after 30 min. the supernatant liquid was eliminated. The precipitate was heated thrice for 1-5 min. in 350 mlof methanol on a water-bath. Each time the methanol was decanted.

After the last decantation the residue was dissolved by refluxing in 1.7 litres of acetone. Upon cooling, the Zn-salt crystallized from the solution;

Yield 77 PREPARATION 3 ZINC MONOISOI-IEXADECYLPI-IQSPHATE a. In a 2 litre reaction vessel fitted'with stirrer, thermometer and droppingfunnel were placed 459 g (3*.

moles) of phosphorus oxychloride. The phosphorus.

oxychloride was cooled to 0 C and then. 726 g (.33 moles) of isohexadecyl alcohol. (commercially available from Esso Belgium- N.V. Antwerp) were added? d'ropwise in such a: way that the hydrogen chloride formed escaped gradually. As soon. as alliof: the isohexadecyl alcohol was added, the temperature hasrisen to; about 25 C. The mixture was left standing overnight: and then stirredfor a few hours while nitrogen was introduced to remove residual hydrogen chloride.

Yield 1,055- g (98%). I

b. In a litrereaction vessel:fittediwithstirrer, thermometer and droppingfunnel were placed: 2.8.litresof water, 300 ml of hydrochloric: acid and; 1,260rmlof dioxan. The mixture was heated on: a boilingwater-bath to 80 C whereupon a solution of l,0l0gzot thie,productformed in step (a) in 1,250 ml of dioxan was added dropwise over 90; minutes. The temperature rose to 9 2- C and the mixture was stirred for 5 hours. at thisteme perature. The mixture was cooled and the. oil separated and agitated with benzene. After concentration: on awater-bath of 75 C, 890 g (9 8? of isohexadecyl phosphate was. formed.

c. 1 mole of isohexadecyl phosphate was dissolved in methanol whereupon a solution of 1 mole of zinc acetate 2-water in 750 ml of methanol was added. The precipitate formed was separated and then stirred several times with warm methanol. T he product was. heated on a water-bath to remove all methanol and then recrystallized from ethylene glycol monomethyl The metal salts of the half-esters of aliphatic dicarboxylic acids can be prepared in a similar way. A stoichiometric amount of the half-ester is dissolved in acetone and the solution obtained is added to a solution of a metal acetate in an aliphatic alcohol. The metal salt of the half-ester separates byprecipitation in the medium. The half-esters used can be prepared,'e.g., by refluxing the cyclic dicarboxylic anhydride and the alco- PREPARATION 4 ZINC DODECYL SUCCINATE 0.05 mole of succinic anhydride and 0.05 mole of dodecyl alcohol were refluxed for 5 hours-in 100 mlof toluene. The solution was concentrated by evaporation and the residue solidified at room temperature. The monolaurylsuccinic acid'was recrystallized from ml of hexane. s

A- solution of 0.02 mole of monolauryl succinic acid in 100 mlof acetone-was mixed with a solution of 0.01 mole of zinc acetate in 20 ml of ethanol. The mixture was leftstanding overnight whereupon the precipitate formed was filtered by suction, washed with ethanol, acetone andether and dried in vacuum.

Melting point 124 C. In a similar way were prepared: zinc decyl succinate (m.p. 123 C), zinc undecyl succinate (m.p. 121 C), zinc tetradecyl succinate (m.p. 123 C), zinc hexadecyl succinate (m.p. 124 C), zinc octadecylsuccinate (m.p. 123C), zinc docosyl succinate (m.p. 123 C),

and

ceptable maximum thickness of the coating of metal salt. Whenthe metal salt is first applied to a treating device,,which is'then brought into contact with the imagingsurface, the amount of metal salt may also vary within wide limits and depends on the manner in which it is applied to the device, on the nature of the particular device employed, on the length of contact between the treating device'and thephotoconductive surface etc. Generally, satisfactory results are obtained when from 0.005 g to 5 g, preferably from 0.05 to 2 g, of metal salt is present persq.m.'

When applying the salt(s') to an applicator member or device which can be .used for simultaneously cleaning the imaging surface the said member or device preferably comprises a fibrous woven or non-woven material as this promotes uniformity of treatment a said fibrous treatment device or member may be of any suitable shape such as in the form of a continuous web, cylinder or belt. Suitable fibrous materials include paper tissue, furs, natural fibres such as cotton, flannel, wool derivatives.

The following examples illustrate the present invention.

EXAMPLE 1 An electrophotographic dry developing mixture comprising about 99 parts by-weight of glass carrier particles with a particle size between 0.6 and 0.8 mm and about 1 part by weight of toner particles, which have a grain size comprised between 2 and 30 microns and have been prepared from 8 parts by weight of polystyrene having a molecular weight of 30,000, 1 part by weight of polyvinylbutyral comprising in addition to vinylbutyral groups 20 by weight of vinyl alcohol groups and 2.5 by weight of vinyl acetate groups, and 1 part by weight of carbon black, was cascaded across a selenium surface bearing an electrostatic image. The developed image was transferred by electrostatic means to a sheetof paper whereon it was fused by heat.

A conventional continuous cleaning web as used in xerographic copying apparatus was impregnated with a l solution of zinc octadecylsuccinate in dimethyl formamide and dried. The cleaning web thus treated was used to remove the residual powder on the photoconductive selenium surface after transfer of the developed image. f

After the copying process had been repeated several times wherein use was made each time of the cleaning EXAMPLE 2 A dispersion of 0.5 g of (H .,,c,,.occH,cHcoo ,zn

(LWHW in 100 ml of ISOPAR G (trade name for an isoparafinic hydrocarbon mixture having a boiling range of l60l 77 C sold by Esso Belgium N.V., Antwerp Belgium) was sprayed on a conventional cleaning web as used in xerographic copying apparatus. The 100 m] was used to cover 0.5 sq.m. When the solvent had evaporated, the web was introduced in the copying apparatus to clean the selenium surface after development of an electrostatic image and transfer of the developed image to a sheet of paper as described in example 1.

By examination of the copies produced and the photoconductive selenium surface for quality and wear respectively, as compared with the use of a non-treated cleaning web it was found that the copies showed superior quality (higher density for same fog value) and that the selenium drum showed less' scratches.

The same favourable results were obtained when the above Zn-salt was applied from a dispersion in nhexane, isododecane, and ISOPAR H (trade name ofor an isoparafinic hydrocarbon mixture having a boiling range of l77-l 88 C sold by Esso Belgium N.V., Antwerp, Belgium).

EXAMPLE 3 Example 1 was repeated with the difference that the cleaning web was not treated with a zinc salt as described in example 1.

For the purpose of treating the cleaning web with a zinc salt in accordance with the present invention, zinc dodecyl succinate was compressed to form a rod which was positioned in the xerographic copying apparatus so that the cleaning web came into contact with the rold of zinc salt before rubbing against the selenium drum for cleaning purposes.

By treatment of the cleaning web with the zinc dodecyl succinate the copies were of superior quality than when using a cleaning web which did not come into contact with the zinc salt before rubbing against the. selenium drum. The imaging surface also showed less wear.

EXAMPLE 4 Zinc docosyl succinate was compressed to form a rod. The rod was rubbed against a selenium drum of a xerographic copying apparatus, so as to leave thereon a deposit of the zinc dococyl succinate, each time bcfore electrostatically charging the selenium drum.

When the copying process comprising the steps of: treating the selenium drum with a rod of zinc docosyl succinate, electrostatically charging the drum, imagewise exposing the selenium surface, developing the electrostatic image formed 'e.g. as described in example 1 and transferring the developed image to a sheet of paper, was repeated several times it was found that the copies produced were of superior quality and the imaging surface showed less wear'than when an untreated selenium drum was used.

We claim:

1. In an electrophotograp'hic imaging process comprising the steps of forming a latent electrostatic image on an imaging surface, forming a powder image corresponding to said electrostatic image by applying a developing material to said surface, transferring said powder image from said imaging surface to a receptor surface, and cleaning said imaging surface, the improvement of applying to said surface not later than during said cleaning at least one member of the group consisting of a hydrophobic bivalent or trivalent metal salt of a half ester of a branched-chain or straight-chain aliphatic dicarboxylic acid containing an aliphatic hydrocarbon group of at least five carbon atoms and a hydrophobic bivalent or trivalent metal salt of a monoester or diester of a phosphorus oxyacid containing an aliphatic hydrocarbon group of at least five carbon atoms.

2. A process according to claim 1 wherein said metal salt is applied by depositing the same on said imaging surface prior to theformation of said electrostatic latent image, whereby said salt remains on said surface during the formation and development of such latent image.

3. A process according to claim 1 comprising applying said metal salt to said imaging surface subsequent to the transfer of the powder image to a receptor surface.

4. A process according to claim 3 comprising applying said metal salt to said. imaging surface and cleaning such surface simultaneously.

5. A process according to claim 1 wherein said metal salt is applied to the imaging surface by wiping said surface for a certain period of time with an applicator member carrying such salt on at least the side thereof which contacts such surface.

6. A process according to claim 5 wherein said applicator member comprises a fibrous web which makes contact with said imaging surface.

7. A process according to claim 5 wherein before contact of the surface of said applicator member with the imaging surface, said applicator surface carries said metal salt in an amount between 0.005 g and 5 g per sq.m.

8. A process according salt is a zinc salt.

9. A process according to claim 1 wherein said aliphatic chain is a hydrocarbon group containing at least 10 carbon-atoms.

to claim 1 wherein said metal 10. A process according to claim 9 wherein said aliphatic chain is a hydrocarbon 'group present in the ester portion of the compound or as a substituent on the radical linking the two carboxylic acid groups of the dicarboxylic acid. I

11. A process according to claim 1 wherein said salt is a salt of a half ester of succinic acid.

12. In an electrophotographic imaging process comprising forming an electrostatic image on a surface, developing said image by means of powder, and subsequently transferring said powder image from said surface to a receptor material, the improvement wherein v the surface on which said electrostatic image is formed carries a deposit thereon of at least one compound of the group consisting of a bivalent or trivalent metal salt of a half ester of a branched-chainor straight-chain aliphatic'dicarboxylic acid containing an aliphatic hydrocarbon group of at least five carbon atoms and a bivalent or trivalent metal salt of a mono-ester or di-ester of a phosphorus oxyacid containing an aliphatic hydrocarbon group of at least five carbon atoms.

13. An electrophotographic imaging process according 00 claim 12 wherein said metal salt is a zinc salt.

14. An electrophotographic imaging process according to claim 12 wherein said aliphatic chain is a hydrocarbon group of at least 10 carbon atoms.

15. An electrophotographic imaging process according to claim 12 wherein said aliphatic chain is a hydrocarbon group of at least five carbon atoms which'is present in the ester portion of the compound or as a substituent on the radical linking the two carboxylic acid groups of the dicarboxylic acid.

' 16. An electrophotographic imaging process according to claim 12 wherein said salt is a salt of a half ester I of succinic acid. 

2. A process according to claim 1 wherein said metal salt is applied by depositing the same on said imaging surface prior to the formation of said electrostatic latent image, whereby said salt remains on said surface during the formation and development of such latent image.
 3. A process according to claim 1 comprising applying said metal salt to said imaging surface subsequent to the transfer of the powder image to a receptor surface.
 4. A process according to claim 3 comprising applying said metal salt to said imaging surface and cleaning such surface simultaneously.
 5. A process according to claim 1 wherein said metal salt is applied to the imaging surface by wiping said surface for a certain period of time with an applicator member carrying such salt on at least the side thereof which contacts such surface.
 6. A process according to claim 5 wherein said applicator member comprises a fibrous web which makes contact with said imaging surface.
 7. A process according to claim 5 wherein before contact of the surface of said applicator member with the imaging surface, said applicator surface carries said metal salt in an amount between 0.005 g and 5 g per sq.m.
 8. A process according to claim 1 wherein said metal salt is a zinc salt.
 9. A process according to claim 1 wherein said aliphatic chain is a hydrocarbon group containing at least 10 carbon atoms.
 10. A process according to claim 9 wherein said aliphatic chain is a hydrocarbon group present in the ester portion of the compound or as a substituent on the radical linking the two carboxylic acid groups of the dicarboxylic acid.
 11. A process according to claim 1 wherein said salt is a salt of a half ester of succinic acid.
 12. In an electrophotographic imaging process comprising forming an electrostatic image on a surface, developing said image by means of powder, and subsequently transferring said powder image from said surface to a receptor material, the improvement wherein the surface on which said electrostatic image is formed carries a deposit thereon of at least one compound of the group consisting of a bivalent or trivalent metal salt of a half ester of a branched-chain or straight-chain aliphatic dicarboxylic acid containing an aliphatic hydrocarbon group of at least five carbon atoms and a bivalent or trivalent metal salt of a mono-ester or di-ester of a phosphorus oxyacid containing an aliphatic hydrocarbon group of at least five carbon atoms.
 13. An electrophotographic imaging process according to claim 12 wherein said metal salt is a zinc salt.
 14. An electrophotographic imaging process according to claim 12 wherein said aliphatic chain is a hydrocarbon group of at least 10 carbon atoms.
 15. An electrophotographic imaging process according to claim 12 wherein said aliphatic chain is a hydrocarbon group of at least five carbon atoms which is present in the ester portion of the compound or as a substituent on the radical linking the two carboxylic acid groups of the dicarboxylic acid.
 16. An electrophotographic imaging process according to claim 12 wherein said salt is a salt of a half ester of succinic acid. 